Subchronic/chronic toxicity of a mixture of four chlorinated dibenzo-p-dioxins in rats. I. Design, general observations, hematology,and liver concentrations

The effects of hexachloronaphthalene on selected parameters of heme biosynthesis and systemic toxicity in female wistar rats after 90-day oral exposure

Hexachloronaphthalenes (HxCNs) are the most toxic congeners of polychlorinated naphthalenes, a group of compounds lately included into the list of persistent organic pollutants (POPs). This study presents the effects of 90-day intragastric administration of HxCN to female Wistar rats at doses of 0.03, 0.1, and 0.3 mg/kg body weight. The study examined selected parameters of the heme synthesis pathway, oxidative stress, hepatic cytochromes level, and basic hematology indicators. A micronucleus test was also performed. The subchronic exposure of rats to HxCN resulted in disruption of heme biosynthesis, hematological disturbances, and hepatotoxicity. The highest dose of HxCN inhibited aminolevulinic acid dehydratase (ALA-D) and uroporphyrinogen decarboxylase (URO-D). Accumulation of higher carboxylated porphyrins in the liver and increased excretion of 5-aminolevulinic acid in the urine was observed after a dose of 0.1 mg/kg body weight. The most sensitive effect of HxCN in rats was very strong induction of hepatic CYP1A1 activity, which was observed after the lowest dose. The highest dose of HxCN induced significant thrombocytopenia, thymic atrophy and hepatotoxicity, expressed as hepatomegaly and hepatic steatosis.

Aryl hydrocarbon receptor knockout rats are insensitive to the pathological effects of repeated oral exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin

Sustained activation of the aryl hydrocarbon receptor (AHR) is believed to be the initial key event in AHR receptor-mediated tumorigenesis in the rat liver. The role of AHR in mediating pathological changes in the liver prior to tumor formation was investigated in a 4-week, repeated-dose study using adult female wild-type (WT) and AHR knockout (AHR-KO) rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Beginning at 8 weeks of age, AHR-KO and WT rats were dosed by oral gavage with varying concentrations of TCDD (0, 3, 22, 100, 300 and 1000 ng kg(-1)  day(-1) ). Lung, liver and thymus histopathology, hematology, serum chemistry and the distribution of TCDD in liver and adipose tissue were examined. Treatment-related increases in the severity of liver and thymus pathology were observed in WT, but not AHR-KO rats. In the liver, these included hepatocellular hypertrophy, bile duct hyperplasia, multinucleated hepatocytes and inflammatory cell foci. A loss of cellularity in the thymic cortex and thymic atrophy was observed. Treatment-related changes in serum chemistry parameters were also observed in WT, but not AHR-KO rats. Finally, dose-dependent accumulation of TCDD was observed primarily in the liver of WT rats and primarily in the adipose tissue of AHR-KO rats. The results suggest that AHR activation is the initial key event underlying the progression of histological effects leading to liver tumorigenesis following TCDD treatment. Copyright © 2015 John Wiley & Sons, Ltd.

Toxicological profile of ultrapure 2,2',3,4,4',5,5'-heptachlorbiphenyl (PCB 180) in adult rats

PCB 180 is a persistent non-dioxin-like polychlorinated biphenyl (NDL-PCB) abundantly present in food and the environment. Risk characterization of NDL-PCBs is confounded by the presence of highly potent dioxin-like impurities. We used ultrapure PCB 180 to characterize its toxicity profile in a 28-day repeat dose toxicity study in young adult rats extended to cover endocrine and behavioral effects. Using a loading dose/maintenance dose regimen, groups of 5 males and 5 females were given total doses of 0, 3, 10, 30, 100, 300, 1000 or 1700 mg PCB 180/kg body weight by gavage. Dose-responses were analyzed using benchmark dose modeling based on dose and adipose tissue PCB concentrations. Body weight gain was retarded at 1700 mg/kg during loading dosing, but recovered thereafter. The most sensitive endpoint of toxicity that was used for risk characterization was altered open field behavior in females; i.e. increased activity and distance moved in the inner zone of an open field suggesting altered emotional responses to unfamiliar environment and impaired behavioral inhibition. Other dose-dependent changes included decreased serum thyroid hormones with associated histopathological changes, altered tissue retinoid levels, decreased hematocrit and hemoglobin, decreased follicle stimulating hormone and luteinizing hormone levels in males and increased expression of DNA damage markers in liver of females. Dose-dependent hypertrophy of zona fasciculata cells was observed in adrenals suggesting activation of cortex. There were gender differences in sensitivity and toxicity profiles were partly different in males and females. PCB 180 adipose tissue concentrations were clearly above the general human population levels, but close to the levels in highly exposed populations. The results demonstrate a distinct toxicological profile of PCB 180 with lack of dioxin-like properties required for assignment of WHO toxic equivalency factor. However, PCB 180 shares several toxicological targets with dioxin-like compounds emphasizing the potential for interactions.

Some clinico and histopathological changes in female goats experimentally exposed to dioxin

Female Baladi goats were used for investigating the toxicological effects of dioxin. Each animal in the treated group was given an oral dose of 4 mL of stock standard solution of dioxin (labelled and native congeners) diluted in 5 mL distilled water (1/3 of LD50) for three times with 2 days interval and slaughtered 16 days post treatment. Blood and tissue samples were taken and subjected for haemogram, biochemical and pathological studies as well as for determination of dioxin residues. Results revealed that exposure of female goats to dioxin induced anemia, leucocytopenia, neutropenia and eosinophilia with non significant increases in activities of serum ALT and AST as compared with untreated group. Meanwhile, activity of ALP and BUN concentration were significantly increased. Histopathological examination showed degenerative and necrotic changes associated with inflammatory reaction in liver and kidney, in addition to cystic glandular hyperplasia and adenomyosis in uterus. In ovarian tissue, marked decrease ofpreantral follicles together with cystic atretic follicle were noticed. The average percentage residues ofpg WHO-TEQ values for dioxins (PCDDs and PCDFs) in liver, kidney, mammary gland, uterus and milk after oral dose were 0.013, 0.0012, 0.0012, 0.009 and 0.0012%, respectively. It was concluded that oral exposure to dioxin in female goats induced adverse effects on liver and kidney. Dioxins had estrogenic like effect as indicated by uterine and ovarian histopathological changes.

Comparative temporal toxicogenomic analysis of TCDD- and TCDF-mediated hepatic effects in immature female C57BL/6 mice

Temporal analyses were performed on hepatic tissue from immature female C57BL/6 mice in order to compare the gene expression profiles for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 2,3,7,8-tetrachlorodibzofuran (TCDF). Time course studies conducted with a single oral dose of 300 microg/kg TCDF or 30 microg/kg TCDD were used to compare differential gene expression on complementary DNA microarrays containing 13,361 features, representing 8194 genes at 2, 4, 8, 12, 24, 72, 120, and 168 h. One hundred and ninety-five genes were identified as differentially regulated by TCDF, of which 116 genes were in common with TCDD, with 109 exhibiting comparable expression profiles (correlation coefficients > 0.3). In general, TCDF was less effective in eliciting hepatic vacuolization, and differential gene expression compared with TCDD when given at an equipotent dose based on a toxic equivalence factor (TEF) of 0.1 for TCDF, especially 72-h postadministration. For example, the induction of Cyp1a1 messenger RNA by TCDF was less when compared TCDD. Moreover, TCDF induced less severe hepatocyte cytoplasmic vacuolization consistent with lower lipid accumulations which significantly subsided by 120 and 168 h when compared with TCDD. TCDF-elicited responses correlated with their hepatic tissue levels which gradually decreased between 18 and 168 h. Although both compounds elicited comparable gene expression profiles, especially at early time points, the TCDF responses were generally weaker. Collectively, these results suggest that the weaker TCDF responses could be attributed to differences in pharmacokinetics. However, more comprehensive dose-response studies are required at optimal times for each end point of interest in order to investigate the effect of pharmacokinetic differences on relative potencies that are important in establishing TEFs.

Chronic toxicity and carcinogenicity of 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin displays a distinct dose/time toxicity threshold (c×t=k) and a life-prolonging subthreshold effect

Chronic toxicity of 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) including its carcinogenicity was studied in female Sprague-Dawley rats in lifetime experiments. Six single dose and three multiple dose rate experiments were conducted with a single dose corn oil control group and a multiple dose rate corn oil control group, respectively. The lowest dose (1.0 mg/kg) of HpCDD and multiple dose rates of corn oil (4.0 ml/kg every other week) both prolonged the life of rats by about 2 months over that of single dose corn oil controls. Higher doses resulted in a predictable shortening of the life of rats after single dose administrations as well as after multiple dose rate administrations. The c x t = k paradigm previously validated for acute toxicity [Toxicol. Sci. 49 (1999) 102] was confirmed for chronic toxicity including carcinogenicity of HpCDD. The c x t = k product was independent of dosing regimen. Anemia and squamous cell carcinoma of the lungs were the earliest and most prevalent endpoints of toxicity. A dose of 2.1 mg/kg and 3.1 mg/kg of HpCDD caused 16.6% and 73.3% lung cancer, respectively. Liver cancer had a low prevalence and was a very late effect occurring only at doses lethal acutely for most rats in the three highest dosage groups. There was no correlation in the dose-dependence of non-malignant hepatic lesions and liver cancer.

Human exposure to dioxins from food, 1999-2002

In response to aggressive attempts to control dioxin emissions over the last 35 years, human exposures to dioxins from the environment have declined significantly. The primary source of human exposure to dioxins at present is food. The sources of dioxins in food are not well understood and are probably varied. Data on the levels of dioxins measured in various foods for samples collected from 2000 to 2002 have recently been released by the US Food and Drug Administration as part of its Total Diet Study. Data on samples collected in 1999, and released in 2002, are also available. Based on those data and on the US Department of Agriculture's most recent food consumption survey (1994-1996 & 1998 Continuing Survey of Food Intakes by Individuals), estimates of dioxin intake for the total US population and for three age groups of children were obtained. Results show that the most recent mean dietary exposures for all groups are below 2 pg TEQ/kg BW/day, the tolerable daily intake established for dioxins by the World Health Organization. Between 1999 and 2002 mean dioxin intakes from food appear to have decreased, but when estimates are adjusted based on a standardized limit of detection and evaluating only those {congenerxfood} combinations common to all 4 years, no trend is apparent. When dioxin concentrations below the limit of detection are represented by one-half the limit, approximately 5% of the intake estimates for 2-year-olds and 1% of the intake estimates for 6-year-olds exceed the tolerable daily intake by about 10%, although such upper-percentile estimates should not be equated with excess risk. When non-detectable dioxin values are set to zero (i.e., when only dioxin values actually measured are used), only 1% of intake estimates exceed the tolerable daily intake for 2-year-olds. As expected, about 50% of daily dietary dioxin intake by the total US population is attributable to meat and dairy products, based on the same food group classifications used by the National Academy of Sciences' Committee on the Implications of Dioxin in the Food Supply. This information may be useful for targeting future risk management activities.

Protein kinase C delta is activated in mouse ovarian surface epithelial cancer cells by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

Interactions between the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and protein kinase C (PKC) signaling pathways are governed in cell and tissue-specific manners, albeit the physiological significance of which is unclear. This research sought to define the effects of TCDD on the PKC pathway using a mouse ovarian surface epithelial cancer cell line (ID8). Phorbol-12-myristate-13-acetate (PMA) potentiated (1 nM) TCDD-induced 7-ethoxyresorufin-O-deethylase (EROD) activity after 24h of treatment, and pre-treatment with (1 microM) of either a general PKC inhibitor (BisI) or PKCdelta-specific inhibitor (Rotterlin) abolished the potentiation indicating that activation of PKC enhances TCDD signal transduction. Western blot analysis revealed that unstimulated ID8 cells express PKCalpha, beta, epsilon, tau, lambda and RACK1. PKCgamma, eta, theta and DGKtheta were not detected. TCDD (1 nM) increased PKCdelta protein approximately eight-fold after 24h of treatment and this effect was dose-dependent (0.1-100 nM); other PKC isoforms and related signaling proteins tested were unaffected by TCDD treatment. Immunofluorescent microscopy revealed that TCDD (1 nM) promoted the subcellular redistribution of PKCdelta, from the cytoplasm and the nucleus to the perinuclear area after 2h of treatment, however, after 24h of treatment PKCdelta was observed in nuclear structures that resembled nucleoli. TCDD (1 nM) also increased total PKC and PKCdelta-specific kinase activities in biphasic time-responsive manners. Total PKC and PKCdelta-specific activities increased after 1-2h of treatment. Then TCDD increased the total PKC activity again after 12h of treatment, whereas, PKCdelta-specific activity resurged at 24h and remained elevated at 48 h after treatment. The results indicate that TCDD preferentially induces PKCdelta protein expression and phosphotransferase activity, and its membrane translocation, indicating a potential intracellular role for PKCdelta as an effector molecule for TCDD-mediated biological events in this ovarian cancer cell line.

Persistent hematologic and immunologic disturbances in 8-year-old Dutch children associated with perinatal dioxin exposure

Perinatal exposure to Dutch "background" dioxin levels in 1990 was high, but comparable with that of other industrialized Western European countries. Exposure during the sensitive perinatal period may cause permanent disturbances. Therefore, we assessed the health status and various hematologic and immunologic parameters among our longitudinal cohort. A medical history was taken and venipuncture performed in a longitudinal cohort of 27 healthy 8-year-old children who had documented perinatal dioxin exposure. Linear regression revealed a decrease in allergy in relation to prenatal (p = 0.02) and postnatal (p = 0.03) dioxin exposure. Increases in CD4+ T-helper cells (p = 0.006) and in CD45RA+ cells (p = 0.02) were seen in relation to postnatal exposure. A persistently decreased platelet count (p = 0.04) and increased thrombopoietin concentration (p = 0.03) were seen in relation to postnatal exposure. This follow-up has shown a decrease in allergy, persistently decreased thrombocytes, increased thrombopoietin, and increased CD4+ T-helper and increased CD45RA+ cell counts. This study provides indications of effects at the stem cell level of perinatal dioxin exposure, persisting until minimally 8 years after birth.

Subchronic toxicity of Baltic herring oil and its fractions in the rat II: Clinical observations and toxicological parameters

This study aimed to increase the knowledge about the toxicity of fish-derived organohalogen pollutants in mammals. The strategy chosen was to separate organohalogen pollutants derived from Baltic herring (Clupea harengus) fillet, in order to obtain fractions with differing proportions of identified and unidentified halogenated pollutants, and to perform a subchronic toxicity study in rats, essentially according to the OECD guidelines, at three dose levels. Nordic Sea lodda (Mallotus villosus) oil, with low levels of persistent organohalogen pollutants, was used as an additional control diet. The toxicological examination showed that exposure to Baltic herring oil and its fractions at dose levels corresponding to a human intake in the range of 1.6 to 34.4 kg Baltic herring per week resulted in minimal effects. The spectrum of effects was similar to that, which is observed after low-level exposure to pollutants such as chlorinated dibenzo-p-dioxins and dibenzofurans (CDD/F) and chlorinated biphenyls, despite the fact that these contaminants contribute to a minor part of the extractable organically bound chlorine (EOCI). The study confirmed previous findings that induction of hepatic ethoxyresorufin deethylase (EROD) activity takes place at daily intake levels 0.15 ng fish-derived CDD/F-TEQs/kg body weight. The study also demonstrated that hepatic vitamin A reduction takes place at somewhat higher daily exposure levels, i.e. 0.16-0.30 ng fish-derived CDD/F-TEQs/kg body weight. Halogenated fatty acids, the major component of EOCI, could not be linked to any of the measured effects. From a risk management point of view, the study provides important new information of effect levels for Ah-receptor mediated responses following low level exposure to organohalogen compounds from a matrix relevant for human exposure.

Induction of oxidative stress in the tissues of rats after chronic exposure to TCDD, 2,3,4,7,8-pentachlorodibenzofuran, and 3,3',4,4',5-pentachlorobiphenyl

The abilities of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,4,7,8-pentachlorodibenzofuran (PeCDF), 3,3',4,4',5-pentachlorobiphenyl (PCB126), and mixtures of these xenobiotics (toxic equivalents, TEQs) to induce oxidative stress in hepatic and brain tissues of rats have been investigated after chronic (30 wk) exposure to these congeners. TCDD, PeCDF, PCB126, and TEQs were administered daily to groups of rats at doses that corresponded to their toxic equivalency factors (TEFs), and the biomarkers of oxidative stress, including the production of superoxide anion, lipid peroxidation, and DNA single-strand breaks (SSBs), were determined in hepatic and brain tissues at the end of the exposure period. The three chemicals caused similar dose-dependent increases in the production of superoxide anion, lipid peroxidation, and DNA SSBs, which plateaued at certain dose ranges, followed by secondary increases at the higher dose levels. Similar effects were also produced by the TEQs; however, the dose-dependent increases in the biomarkers of oxidative stress were continuous and never achieved plateau levels. Except for PCB126, where statistical analyses revealed greater productions of superoxide anion and lipid peroxidation in brain tissues as compared with hepatic tissues, no significant differences were revealed between the two tissues in response to the other xenobiotics or the TEQs. Nonsignificant differences were also revealed when comparing the effects induced by the TEQs with those induced by the individual chemicals.

Use of Haber's Rule to estimate the risk of diabetes from background exposures to dioxin-like compounds

Recent publications have reported a weak but consistent correlation between diabetes incidence and occupational or accidental exposure to dioxins. As with most work involving environmental xenobiotics, these studies suffered from the analytical problem that the reference populations had some degree of exposure. We have used Haber's Rule to relate the integrated exposure of subjects involved in an industrial exposure to dioxins, reported by Sweeney et al. [Teratog. Carcinog. Mutagen. 17 (1998) 241], to the incremental probability of diabetes incidence. We estimated that background exposure to dioxin-like compounds by the referents contributed <1% of their diabetes risk, suggesting that background exposure to dioxins is not a significant risk factor for individuals who have not been occupationally or accidentally exposed.

Structure-activity relationships and dose responses of polychlorinated dibenzo-p-dioxins for short-term effects in 2,3,7,8-tetrachlorodibenzo-p-dioxin-resistant and -sensitive rat strains

Dose responses of the characteristic short-term effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin (HxCDD), and 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) were compared in the resistant Han/Wistar (Kuopio) (H/W) rats and the sensitive Long-Evans (Turku/AB) (L-E) rats. The resistance of H/W rats is linked to the altered H/W-type aryl hydrocarbon receptor (AHR). Exceptionally, in terms of acute lethality, the most potent congener for H/W rats is HxCDD, followed by HpCDD, PeCDD, and TCDD. The study objectives were to find out if this exceptional sensitivity of H/W rats also holds for nonlethal toxic endpoints and to compare potency and efficacy (magnitude of effect) of PCDDs between L-E and H/W rats. Dose responses for several endpoints were determined, modeled, and used for ED50 and relative potency (REP) calculations. For all endpoints measured, TCDD was the most potent congener, followed by PeCDD, HxCDD, and HpCDD in both strains, and the REP estimates were consistent with the current toxic equivalency factors (TEFs). For most endpoints, H/W rats showed smaller responses to all congeners than L-E rats, and this difference was due to lower efficacy rather than lower potency. H/W rats showed lower efficacy to body weight loss, serum aspartate aminotransferase activity, and serum concentrations of total bilirubin, free fatty acids, and thyroxine. In contrast, effects on cytochrome P4501A1 induction, thymus atrophy, and dental defects were similar in both strains. In conclusion, the results are in agreement with the current WHO-TEFs and imply that relative potency values derived from mortality are not necessarily valid for other endpoints. The results support our previous observations about two different types of AHR-mediated mechanisms. Type I effects are similar in both strains, and type II effects show decreased efficacy of toxic response in relation with the altered H/W-type AHR.

Persistent, low-dose 2,3,7,8-tetrachlorodibenzo-p-dioxin exposure: effect on aryl hydrocarbon receptor expression in a dioxin-resistance model

Most toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) are mediated by the aryl hydrocarbon receptor (AHR). A single, acute dose of TCDD can alter its own receptor levels thus complicating evaluation of dose-response relationships for AHR-mediated events. Since environmental exposure to dioxins is typically of a repeated low-dose nature, we examined the effect of such exposure on AHR expression. Three rat strains differing greatly in their sensitivity to acute TCDD lethality, Long-Evans (Turku AB) (L-E) (LD50 approximately 10 microg/kg); Sprague Dawley (SD) (LD50 approximately 50 microg/kg); and Han/Wistar (Kuopio) (H/W) (LD50 > 9600 microg/kg), were administered TCDD intragastrically, biweekly for 22 weeks producing doses equivalent to 0, 10, 30, and 100 ng/kg/day. Changes in hepatic AHR levels were quantitated at the protein level by radioligand binding and immunoblotting and at the mRNA level by RT-PCR. Cytosolic AHR protein was elevated at 10 or 30 ng/kg/day TCDD in SD and L-E rats; AHR mRNA was also elevated at these doses, suggesting a pretranslational mechanism. There was no apparent relationship between TCDD-induced AHR regulation and strain sensitivity to TCDD. Overall, "subchronic" TCDD did not greatly perturb AHR expression. The maintenance of relatively constant receptor levels in the face of persistent agonist stimulation is in contrast to the sustained depletion of AHR by TCDD observed in cell culture and to the fluctuations in AHR observed hours to days following acute TCDD exposure in vivo. Changes in AHR levels may affect dose-response relationships; the effect of TCDD on its own receptor at environmentally relevant dosing schemes is therefore important to risk assessment.

2,3,7,8-tetrachlorodibenzo-p-dioxin decreases responsiveness of the hypothalamus to estradiol as a feedback inducer of preovulatory gonadotropin secretion in the immature gonadotropin-primed rat

Sprague-Dawley rats (23-day-old) were dosed with TCDD (32 microg/kg) in corn oil or vehicle alone. Equine chorionic gonadotropin (eCG) was injected (5 IU, sc) 24 h later to induce follicular development. Another 24 h later, half of TCDD- or corn oil-treated rats were injected (sc) with 17 beta-estradiol-cypionate (ECP, at 0.004 to 0.5 mg/kg). Blood and ovaries were collected on expected proestrous (preovulatory period) at 51, 54, and 58 h after eCG injection as well as in the morning after ovulation (72 h after eCG). Serum concentrations of 17 beta-estradiol (E), progesterone (P), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) were determined by radioimmunoassay. The number of ova shed was measured at 72 h after injection of eCG by irrigating ova from oviducts. During the preovulatory period (approximately 58 h after eCG injection), a circulating level of 70-100 pg E/ml coincided with LH and FSH surges and later normal ovulation of 10 to 12 ova/rat was observed in controls. However, the same concentration of E was not associated with LH and FSH surges in rats treated with TCDD (32 microg/kg), resulting in reduced ovarian weight gain and reduction of ovulation by 70 to 80% (2-3 ova/rat). Blockage of the gonadotropin surge, reduced ovarian weight gain, and ovulation were all reversed completely by the lowest effective dose of ECP (0.1 mg/kg). At 72 h after eCG, serum P secretion was reduced and serum E levels were significantly increased compared to those of corn oil-treated controls. ECP alone had no effect on serum P levels at any time point, but in rats treated with TCDD and ECP, both the reduction of P (at 58 and 72 h) and the increase in E secretion (72 h) were completely reversed. Further studies confirmed that restoration by ECP of gonadotropin surges and associated ovulation could not be attained until circulating levels of E rose sufficiently high to trigger the LH and FSH surges. The new action threshold of E for inducing gonadotropin surges in rats treated with TCDD (32 microg/kg) was determined to be eight- to 10-fold higher than that in controls. Thus, it is apparent that TCDD decreased the responsiveness of the hypothalamus to E as a feedback inducer of preovulatory gonadotropin secretion.

The role of time in toxicology or Haber's c x t product

It happened exactly 100 years ago that Warren established for the first time a quantitative link between dose and time while studying the toxicity of sodium chloride in Daphnia magna (Straus). During this century many toxicologists in different contexts returned to this idea, which has become known as Haber's rule of inhalation toxicology. Most attempts to explore this relationship ended in frustration because of the supposed occurrence of exceptions. Thus, toxicologists concentrated on the quantitative relationship between dose and effect under mostly isotemporal conditions while time took a back seat and was assigned such arbitrary, semiquantitative designations as acute, subacute, subchronic and chronic. Time itself as a quantifiable variable of toxicity was seldom studied and when it was studied, it was often not under isodosic (steady state) conditions as required by theory. A recent analysis of toxicological time indicated the impact of three independent time scales (toxicokinetic, toxicodynamic, exposure frequency/duration) in toxicological studies, which interact with dose and effect to yield the enormous complexity known to every toxicologist. Based on prototypical examples when toxicokinetic (dioxins), toxicodynamic (nitrosamines, benzene) or exposure frequency (methylene chloride, chloroacetic acid, HgCl(2), CdCl(2), etc.) represent the critical time scale, the general validity of the c x t=k concept will be discussed as a starting point for a theory of toxicology. As endpoints of toxicity, (delayed) acute toxicity, blood dyscrasias and cancer will be used to illustrate the critical conditions needed to demonstrate the validity of this theory.

Endocrine disruptive effects of polychlorinated aromatic hydrocarbons on intestinal cholecystokinin in rats

The ubiquitous and persistent nature of polychlorinated aromatic hydrocarbons (PCAHs) in our environment and the risk of exposure to PCAHs have provoked concern over their potential toxicity. In humans, exposure to PCAHs is aimed chiefly at epithelial cells residing in the intestinal mucosa, because oral intake of contaminated food is a major source of PCAHs. The purpose of this study, therefore, was to examine the effects of chronic exposure to various PCAHs [i.e. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,4,7,8-pentachlorodibenzofuran (PeCDF), 3,3',4,4',5-pentachlorobiphenyl (PCB-126), and 2,2'4,4'5,5'-hexachlorobiphenyl (PCB-153)], given alone or as mixtures, on intestinal cholecystokinin (CCK) peptide and messenger RNA levels. We show that chronic PCAH treatment significantly lowers intestinal levels of stored CCK peptide. Intestinal CCK messenger RNA levels are not affected. In addition, 3,3',4,4',5-pentachlorobiphenyl treatment increased intestinal insulin-like growth factor-binding protein-3 levels in a dose-related manner. Acute 2,3,7,8-tetrachlorodibenzo-p-dioxin treatment of intestinal CCK cells lowered levels of CCK-processing enzymes (i.e. prohormone convertase-1 and -2). Together, these data indicate that PCAHs may decrease intestinal levels of stored CCK peptide by affecting the intestinal insulin-like growth factor system and CCK processing.

Relative potency values derived from hepatic vitamin A reduction in male and female Sprague-Dawley rats following subchronic dietary exposure to individual polychlorinated dibenzo-p-dioxin and dibenzofuran congeners and a mixture thereof

This study investigated the potency of individual polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) to reduce hepatic vitamin A in the rat. Dose-response relationships were determined following long-term dietary exposure to 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,4,7, 8-pentachlorodibenzofuran, 1,2,3,4,8-pentachlorodibenzofuran, 1,2,3, 7,8-pentachlorodibenzofuran, 1,2,3,6,7,8-hexachlorodibenzofuran, 1,2, 3,7,8-pentachlorodibenzo-p-dioxin, octachlorodibenzo-p-dioxin, octachlorodibenzofuran, or mixtures of some of these congeners. The aim was to estimate vitamin A-related relative potency (REP) values for each congener in relation to that of TCDD and to investigate if these values were in accordance with REP values estimated for the subchronic toxicity observed in the same study. An additional aim was to investigate if the effect on hepatic vitamin A levels was additive compared to the effect of the individual congeners. The obtained results demonstrate that hepatic vitamin A reduction occurs as a consequence of long-term low-level exposure to 2,3,7, 8-substituted but not to non-2,3,7,8-substituted congeners. Female rats were slightly more responsive to this effect as judged from the lower EC50 values for all the congeners in this sex. The vitamin A-related REP values were similar for female and male rats and were in good agreement with the estimated REP values for subchronic toxicity in the same animals. The vitamin A effect of the individual congeners in the mixture tended to be somewhat less than pure additive for male rats and very close to pure additive for female rats. In conclusion, the presented data show that reduction of hepatic vitamin A is a sensitive marker of an altered retinoid homeostasis following long-term low-dose exposure to dioxin-like compounds, which essentially conforms to their assumed additive mechanism of action.

Gonadotropin-releasing hormone (GnRH) partially reverses the inhibitory effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin on ovulation in the immature gonadotropin-treated rat

Several studies have shown that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has inhibitory effects on ovulation. This action may be the result of either direct effect(s) of TCDD on ovarian function or via altered secretion of pituitary luteinizing hormone (LH) and follicle stimulating hormone (FSH) which regulate ovarian follicular development and ovulation. To further evaluate the effects of TCDD on pituitary gonadotropins and their regulation, the potential role of gonadotropin-releasing hormone (GnRH) was investigated in the current study. Immature (23-day-old) female Sprague-Dawley rats were dosed with TCDD (32 microg/kg) in corn oil or vehicle alone. Equine chorionic gonadotropin (eCG) was injected subcutaneously (5 IU, sc) 24 h later to induce follicular development. Immediately prior to the expected time of the LH/FSH surges, 54 h after eCG injection, half of TCDD- or corn oil-treated rats were injected with GnRH (2 microg/rat, sc). Blood and ovaries were collected at 54, 56, 58, 60 and 72 h after eCG. Serum concentrations of 17beta-estradiol (E(2)), progesterone (P(4)), LH, and FSH were determined by radioimmunoassay. An indication of ovulation rate was assessed at 72 h after injection of eCG by irrigating the ova from oviducts. TCDD reduced the number of ova in the oviducts by 70-80% (2-3 ova/rat) and this was confirmed by the number of corpora lutea. GnRH partially restored ovulation (6-7 ova/rat) in TCDD-treated rats without reversing its effect on ovarian weight reduction. In controls, the LH and FSH surges at 58 h after eCG were significantly reduced at that time in TCDD-treated rats. However, in rats treated with TCDD and GnRH, a huge LH/FSH surges occurred at 56 h after eCG injection. GnRH alone enhanced E(2) and P(4) serum levels at 56-58 h after eCG injection. In rats treated with both TCDD and GnRH, E(2) secretion was significantly lower at 58, 60, and 72 h when compared with GnRH alone, whereas serum P(4) was only decreased at 72 h after eCG injection. The results indicate that exogenous GnRH induces LH and FSH surges in TCDD-treated rats, but only partially restores the inhibitory effects of TCDD on ovulation.

Dose and time as variables of toxicity

Recognizing that if there is no exposure, there is no toxicity leads us to the conclusion that if there is exposure, toxicity can ensue when exposure exceeds a certain dose and/or time and that it will be dependent on toxicokinetics and toxicodynamics. Analysis of the fundamental description of toxicity (dT/dE=dT/dDxdD/dKxdK/dE, where T stands for toxicity, D for toxicodynamics, K for toxicokinetics and E for exposure) yields the recognition of three independent time scales, the first being an intrinsic property of a given compound (what does the chemical do to the organism), which is the dynamic time scale. The second time scale is an intrinsic property of a the organism (what does the organism do to the chemical), which represents the kinetic time scale. The frequency of exposure denotes the third time scale, which is independent of the dynamic and kinetic time scales. Frequency of exposure depends on the experimental design or on nature, but not on the organism or substance. A liminal condition occurs when the frequency becomes so high that it is indistinguishable from continuous exposure. Continuous exposure forces the two other time scales to become synchronized thereby reducing complexity to three variables: dose, effect and one time scale. Keeping one of those variables constant allows for the study of the other two variables reproducibly under isoeffective or isodosic or isotemporal conditions. However, any departure from continuous exposure will introduce the full complexity of four independent variables (dose and kinetic, dynamic and frequency time scales) impacting on the effect (dependent variable) at the same time. The examples discussed in this paper demonstrate how nature in the form of long half-lives provides liminal conditions when either kinetic or dynamic half lives force synchronization of all three time scales. However, for compounds having very short dynamic or kinetic half-lives, only continuous exposure will provide a synchronized time scale. A decision tree-type approach is being used to illustrate how to reduce the enormous complexity generated by five variables (dose, effect and up to three time scales) in toxicology to manageable proportions by identifying and modeling the rate-determining (-limiting) step(s) in the manifestation of toxicity.

Effects of polychlorinated dibenzofurans, biphenyls, and their mixture with dibenzo-p-dioxins on ovulation in the gonadotropin-primed immature rat: support for the toxic equivalency concept

Previous studies have shown that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), and 1,2,3,4,7, 8-hexachlorodibenzo-p-dioxin (HxCDD), and their equipotent mixture block ovulation, reduce ovarian weight gain and alter preovulatory hormone levels in a similar manner. The objective of the current experiment was to investigate the effect of other structurally related compounds such as chlorinated furans and biphenyls on ovulation and related hormonal endpoints. The gonadotropin-primed immature female rat model was used to study the effect of 2,3,4,7, 8-pentachlorodibenzofuran (PeCDF), 3,3',4,4',5-pentachlorobiphenyl (PeCB), and 2,2',5,5' tetrachlorobiphenyl (TCB) and their mixture with polychlorinated dibenzo-p-dioxins (PCDDs) on ovulation. Rats were dosed on Day 23 of age at 0900 h with individual congeners (PeCDF, PeCB, TCB) or a mixture of five compounds, which included TCDD, PeCDD, HxCDD, in addition to PeCDF and PeCB. Equine choronic gonadotropin (eCG; 5 IU) was injected 24 h later to induce follicular development. Blood and ovaries were harvested, and ovarian weights determined at various times after eCG. Serum concentrations of 17beta-estradiol (E(2)), progesterone (P(4)), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) were determined by radioimmunoassay. At 72 h after injection of eCG, the number of ova shed was measured by irrigating the ova from oviducts. The slopes of the dose-responses for inhibition of ovulation generated by the individual PeCDF, PeCB, and/or their mixture with PCDDs were similar. PeCDF, PeCB, and the mixture increased serum concentrations of E(2) at 72 h after eCG injection, the day of expected ovulation; in contrast, serum P(4) and FSH were decreased at that same time point. Only the high doses of TCDD, PeCDF, and PeCB blocked LH and FSH surges at 58 h after eCG. The ovarian histology revealed that the effects of PeCDF, PeCB, and the mixture were very similar to those of PCDDs, consisting of ova in large preovulatory follicles and a lack of or reduced number of corpora lutea. Parallel dose-responses of the individual congeners (PeCDF and PeCB) and their equipotent mixture with PCDDs support the toxic equivalency (TEQ) concept for the blockage of ovulation. Thus, PCDDs, PCDFs, and PeCBs appear to block ovulation by the same or a very similar mechanism of action.

Toxic equivalency factors of polychlorinated dibenzo-p-dioxins in an ovulation model: validation of the toxic equivalency concept for one aspect of endocrine disruption

Polychlorinated dibenzo-p-dioxins (PCDDs) are structural analogues, which produce a similar spectrum of biological and toxicological responses in animals, albeit with differential potencies. Very consistent structure-activity relationships have been found for acute toxicity and some biochemical effects among these compounds. For the current experiments, the gonadotropin-primed immature female rat model was used to study the effect of 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD), 1,2,3,7, 8-pentachlorodibenzo-p-dioxin (PeCDD), and 1,2,3,4,7, 8-hexachlorodibenzo-p-dioxin (HxCDD) on ovulation. Single doses of different PCDDs and their mixture were given orally to 23-day-old rats. Gonadotropin from pregnant mare's serum (PMSG) was injected (5 IU) 24 h later to induce follicular maturation. Rats were decapitated at various times after PMSG, blood was collected, and ovarian weight was measured. Serum concentrations of 17beta-estradiol (E2), progesterone (P4), luteinizing hormone (LH), follicle stimulating hormone (FSH), and prolactin (PrL) were determined by radioimmunoassay. Ovulation was measured at 72 h after injection of PMSG by counting ova flushed from oviducts. PCDDs dose dependently decreased the number of ova per ovary and reduced ovarian weight gain induced by PMSG. The slopes of the dose-response curves generated by individual PCDDs and/or their mixture were similar. PMSG-induced increase in serum E2 was enhanced on the day of expected ovulation by PCDDs; in contrast, serum P4 and FSH were decreased at that same time point. PCDDs also altered the temporal pattern of serum E2, FSH, and LH but not that of PrL. Histologically the effect of all three PCDDs consisted of ova trapped in preovulatory follicles and a lack of or reduced number of corpora lutea. The results indicate that the PCDDs, tested in the present model, have the same mode of action on ovulation and the reproductive hormones, e.g., LH, FSH, P4 and E2. Furthermore, the dose responses of the individual congeners are parallel to each other and also to that of their equipotent mixture, which represent a validation of the TEQ concept for one aspect of endocrine disruption, that is for inhibition of ovulation.

Subchronic/chronic toxicity of a mixture of four chlorinated dibenzo-p-dioxins in rats. II. Biochemical effects

Groups of 20 male and 20 female rats were given five different oral doses of a mixture of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 1, 2,3,7,8-pentaCDD, (PCDD) 1,2,3,4,7,8-hexaCDD (HxCDD), and 1,2,3,4,6, 7,8-heptaCDD (HpCDD) divided into four daily loading doses and six biweekly maintenance doses. PCDD and HxCDD were used as positive controls. The dosing period was 13 weeks, after which half of the rats were necropsied and the rest provided with an off-dose period of another 13 weeks. Liver ethoxyresorufin O-deethylase (EROD) activity was dose-dependently increased in rats dosed with the mixture starting at the lowest dose (13- to 16-fold increase), with the effect reaching maximum at the middle dosage (74- to 112-fold increase), as well as in the positive control groups. There was some indication of reversibility at the lower doses and in positive controls during the off-dose period. The activity of phosphoenolpyruvate carboxykinase (PEPCK) in liver was dose-dependently decreased (maximally by 51%). This effect was more distinct in males than in females. Liver tryptophan 2,3-dioxygenase (TdO) activity decreased maximally by 53% at the two highest doses. This effect was more distinct in females than in males. Serum tryptophan concentrations were increased in rats moribund due to wasting. Some reversibility was apparent by the end of the off-dose period regarding all three biochemical markers of CDD toxicity. Serum glucose concentrations were decreased at the three highest doses of the mixture and in positive controls, maximally by 30%, with some reversibility during the off-dose period. There was a dose-dependent decrease of serum thyroxine (T4) concentrations in rats given the mixture and in the PCDD and HxCDD dosage groups (maximally by 69%), with some reversibility in males during the off-dose period. Serum triiodothyronine (T3) levels were not much affected, except that they tended to be decreased in rats moribund with hemorrhage or anemia. The results demonstrate that comparable biochemical changes occur after multiple as after single dosing with CDDs and that TEFs derived from acute studies can be used to predict the toxicity of mixtures of CDDs regardless whether they are administered as single compounds or as a mixture. This study supports the validity of the toxic equivalency factor (TEF) method and the notion of additive toxicity for CDDs as currently used in the risk assessment of these compounds.

Subchronic/chronic toxicity of 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) in rats. Part I. Design, general observations, hematology, and liver concentrations

Groups of 20 male and 20 female adult Sprague-Dawley rats were given five different doses of 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD). Total doses for males and females (30.9/18.5, 370/222, 2222/1333, 6667/4000, and 10000/6000 microg/kg) were divided into four daily loading doses and six biweekly maintenance doses. Positive controls were administered 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD, total dose 70/41.9 microg/kg). Liver concentrations, as determined by GC/MS, reflected quite accurately the calculated dose ratios. The dosing period was 13 weeks, after which half of the rats were necropsied and the rest provided with an off-dose period of another 13 weeks. Body weight gain was dose-dependently reduced throughout the study. Mortality occurred dose-dependently, starting on Day 22 and continuing until the end of the off-dose period. Mortality rates at the end of the off-dose period were 90 and 40% for males and 60 and 10% for females in the two highest dose groups. Clinical signs and necropsy findings suggested that the cause of death was related to wasting (early deaths), gastrointestinal and nasal hemorrhage (between Days 64 and 126), or anemia (late deaths, after Day 111). Prothrombin times were prolonged intermittently, mainly at the highest dose of HpCDD. Platelet counts were dose-dependently decreased at the two highest doses of HpCDD and in the TCDD-treated group. This study demonstrates that the relative potency derived from acute toxicity studies is the same as that observed in this subchronic/chronic toxicity study of HpCDD and TCDD, confirming the validity of 0.007 as the toxic equivalency factor (TEF) for HpCDD, which is in good agreement with the international TEF of 0.01.

Subchronic/chronic toxicity of 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) in rats. Part II. Biochemical effects

Groups of 20 male and 20 female rats were given five different oral doses of 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) or one dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) divided into four daily loading doses and six biweekly maintenance doses. The dosing period was 13 weeks, after which half of the rats were necropsied and the rest assigned to an off-dose period of another 13 weeks. At the end of the dosing period, liver ethoxyresorufin O-deethylase (EROD) activity was dose-dependently increased starting at the lowest dose (7- to 10-fold) with maximum induction (50- to 100-fold) at the middle or second highest dose. There was a slight reversibility of this effect in HpCDD-treated rats, particularly at lower doses, and a pronounced reversibility in TCDD-dosed rats, both in accordance with respective toxicokinetics. The activity of phosphoenolpyruvate carboxykinase in liver was dose-dependently decreased (up to 60%) at the two or three highest doses of HpCDD and also in the TCDD dosage group. Liver tryptophan 2,3-dioxygenase activity was decreased at the two highest doses of HpCDD (up to 41%), particularly in females. Serum tryptophan concentrations were elevated in rats found moribund due to wasting. There was a dose-dependent decrease in serum glucose concentrations (up to 30%) at the end of the dosing period. Serum thyroxin (T4) concentrations showed a dose-dependent decrease (78% at the highest dose) beginning in the middle dose for HpCDD and in the TCDD dosage group. Serum triiodothyronine (T3) concentrations were only slightly affected, except that they were somewhat decreased in moribund animals. The results demonstrate that similar biochemical changes occur in rats after single as after multiple dosing with HpCDD and TCDD. Based on these endpoints, the relative potency of HpCDD after subchronic exposure is in agreement with the international toxic equivalency factor (I-TEF) of 0.01 and, more specifically, with a TEF of 0.007 based on LD50 values in the same strain of rats.